Changes

Changes from Mercurial (hg https://foss.heptapod.net/isa-afp/afp-devel/ default)

added lemmas ex_unify_if_unifiers_not_empty, ex_mgu_if_unifiers_not_empty, and imgu_range_vars_of_equations_vars_subset

Changeset 13183:bab4d1b38467 by desharna: added lemmas ex_unify_if_unifiers_not_empty, ex_mgu_if_unifiers_not_empty, and imgu_range_vars_of_equations_vars_subset
The file was modified	`thys/First_Order_Terms/Unification.thy`